1. Field of the Invention
This invention relates to an audio signal input circuit in a sound recording/reproducing apparatus.
2. Description of the Related Art
An ordinary stereo external microphone is connected to a recording apparatus via a plug having three electrodes. The electrodes are respectively defined, from a plug end, as a left channel (L), a right channel (R) and a ground (GND).
An audio signal from the microphone is input to microphone amplifiers of the right and left channels via the plug, and amplified and then sent to a next stage circuit. In a plug-in power system, power for the microphone is supplied from a microphone power supply circuit of the recording apparatus to the right and left channels of the external microphone respectively via a resistance and an audio signal line.
If a plug of a monaural external microphone is connected to a jack for the stereo external microphone, no signal is input to the right channel amplifier. Sound is recorded only in the left channel, and no sound is recorded in the right channel. This therefore results in wasteful use of a recording medium which records the audio signals, and a sound is output only from the left when reproduced.
Jpn. Pat. Appln. KOKAI Publication No. 2001-333500 (page 4, FIG. 5) discloses an apparatus which determines whether an input audio signal is a stereo sound or monaural sound. In this document, a software stored in a ROM judges whether the input audio signal is stereo or monaural.
A conventional sound recording/reproducing apparatus is not capable of judging whether the microphone connected to a microphone input terminal (microphone jack) is a stereo microphone or monaural microphone. In the document mentioned above, computing processing is performed in an MPU to judge whether the input audio signal is stereo or monaural, but a specific method is not described, and it can not be readily implemented.